1. Field of Technology
The present invention relates to a fiber device for generating a laser light, a wavelength converter for obtaining a stable visible high-output laser light by combining the fiber device and a wavelength conversion element, and an image forming apparatus using this wavelength converter as a light source.
2. Description of the Background Art
Visible light sources having strong monochromaticity and capable of outputting W-class high outputs are necessary in realizing large-scale displays, high-luminance displays and the like. Out of three primary colors of red, green and blue, a red high-output semiconductor laser used in a DVD recorder or the like can be utilized as a small-scale light source having high productivity for a red color. However, for a green or blue light source, realization by means of a semiconductor laser or the like is difficult and there is a demand for a small-scale light source having high productivity. Particularly, it is difficult to obtain a green output light because there is no suitable material that can be constructed as a semiconductor laser, wherefore it is highly difficult to realize a green light source.
Among such light sources, a wavelength converter constructed by combining a fiber laser and a wavelength conversion element is realized as a low-output visible light source. Green and blue small-scale light sources are well-known which utilize a semiconductor laser as a light source for an excitation light for exciting the fiber laser and a nonlinear optical crystal as a wavelength conversion element.
However, several problems need to be solved in order to obtain W-class high-output green and blue lights from such a wavelength converter. FIG. 21 shows a schematic construction of a conventional wavelength converter 220. Based on this construction, the case of obtaining a green output light is described. The wavelength converter 220 shown in FIG. 21 includes a fiber laser 215 for outputting a fundamental wave, a wavelength conversion element 201 for converting the fundamental wave into a green laser light, and a lens 202 for condensing a fundamental wave output to an end surface of the wavelength conversion element 201.
Next, the basic operation of the fiber laser 215 is described. First, in FIG. 21, an excitation light from a laser light source 203 for excitation is incident on one end 204a of a fiber 204. After the incident excitation light is absorbed by a laser active substance contained in the fiber 204, a seed light of the fundamental wave is generated inside the fiber 204. This seed light of the fundamental wave is reflected to reciprocate many times in a laser cavity having a fiber grating 204b formed at the fiber 204 and fiber gratings 205b formed at a fiber 205 different from the fiber 204 as a pair of reflection mirrors. Simultaneously, the seed light is amplified with a gain given by the laser active substance contained in the fiber 204 to increase the light intensity thereof and also has the wavelength thereof selected to reach laser oscillation. It should be noted that the fibers 204 and 205 are connected by a connecting portion 206 and the laser light source 203 is current driven by a laser current source 207 for excitation.
Next, the basic operation of the wavelength converter 220 is described. The fundamental wave is outputted from the fiber laser 215 as described above and is incident on the wavelength conversion element 201 via the lens 202. The fundamental wave from the fiber laser 215 is converted into a harmonic wave by the nonlinear optical effect of the wavelength conversion element 201. This converted harmonic wave is partly reflected by a beam splitter 208, and the harmonic wave having transmitted through the beam splitter 208 becomes a green laser light which is an output light of the wavelength converter 220.
The harmonic wave partly reflected by the beam splitter 208 is utilized by being converted into an electrical signal after being received by a light receiving element 209 for monitoring the output light of the wavelength converter 220. An output controller 210 adjusts a drive current for the laser light source 203 in the laser current source 207 for excitation so that the intensity of this converted signal enables a desired output to be obtained in the wavelength converter 220. Then, the intensity of the excitation light from the laser light source 203 is adjusted, the output intensity of the fundamental wave of the fiber laser 215 is adjusted and, as a result, the output intensity of the wavelength converter 220 is adjusted. In this way, the output intensity of the wavelength converter is kept constant, i.e. a so-called automatic power control (abbreviated as “APC”) stably operates.
It is possible to obtain a green high-output laser light of several hundreds mW by such a construction, but it is difficult to obtain a W-class green high-output laser light. Specifically, the fundamental wave of the fiber laser and the output of the excitation light need to be increased in order to increase the light output of the wavelength converter. However, the fiber laser cannot reach the laser oscillation if the length of the fiber 204 exceeds a certain length since an absorbed amount of the fundamental wave increases in proportion to the length even if an attempt is made to increase the gain of the fundamental wave by extending the length of the fiber 204 in the construction of the fiber laser 215 shown in FIG. 21.
The light absorption of the fundamental wave by the fiber notably increases as the oscillation wavelength of the fiber laser is shortened. As the oscillation wavelength is shortened from 1080 nm to 1020 nm, an optimal fiber length becomes shorter. Thus, an interval during which the seed light is amplified is shortened and the output of the fundamental wave obtained from the fiber laser decreases.
In order to increase the output of the fundamental wave in such a situation, the fiber laser is excited by setting the length of the fiber to a suitable length to increase the output of the excitation light. However, since the length of the fiber is not long enough, the excitation light having a fairly high output intensity remains without being completely absorbed. Accordingly, in the case of obtaining a W-class high output from such a wavelength converter using the fiber laser, a problem of the deterioration of the fiber becomes significant due to an increase of the light output as a sum of the fundamental wave and the excitation light in the fiber and a temperature increase resulting from the increased light absorption. There is another problem that the light output in the fiber increases and the laser light source for excitation is damaged by a return light of the increased fundamental wave.
Various artifices have been made to solve these problems. As an example of preventing the damage of the laser light source, a wavelength selecting filter is inserted in a light path in a fiber laser for light communication so that a signal light amplified in the fiber and having a high peak light output does not return to a laser light source for excitation. Utilizing a small wavelength difference between an excitation light and the signal light, this wavelength selecting filter lets the excitation light transmit therethrough, but reflects the signal light. In this way, only the excitation light is emitted from the laser light source for excitation, and the amplified signal light does not return, wherefore the laser light source is not damaged (see, for example, Japanese Unexamined Patent Publication No. H05-7038).
On the other hand, for the prevention of the fiber deterioration, there is an example in which the structure of a fiber doped with a rare-earth element as a laser active substance is devised. Specifically, in a fiber laser for outputting a fundamental wave having a wavelength in a 3 μm band used in the medical field, the absorption of the excitation light is optimized by determining the range of the doped amount of the rare-earth element or by regulating the diameters of the core and clad of the fiber within specified ranges. By adopting such a structure, a high light output of 3 W is obtained without deteriorating the fiber laser (see, for example, Japanese Unexamined Patent Publication No. 2005-79197).
Further, in the light communication field and the like, a connecting structure of a fiber and a guide fiber when an incident light is incident on the fiber, to which a signal light is transmitted, via the guide fiber is devised to prevent the deterioration of the fiber (see, for example, Japanese Unexamined Patent Publication No. 2005-19540).
Further, a core part of a fiber is covered by an outer core made of a material having a higher refractive index than the core part and, in the case where a light output in the fiber increases, a light is caused to leak to the outside of the fiber to suppress an increase of the light output, thereby preventing the deterioration of the fiber (see, for example, Japanese Unexamined Patent Publication No. 2004-170741).
Although being designed to solve problems different from that of the present application, there have been also proposed a construction for reflecting an excitation light in a fiber to separate an oscillated light and the excitation light (see, for example, Japanese Unexamined Patent Publication No. 2005-109185) and a method for improving an excitation efficiency by controlling the temperature of an excitation laser in an ASE light source device using an Yb fiber (see, for example, Japanese Unexamined Patent Publication No. 2004-64031).
However, the above conventional wavelength converter can obtain only 2 to 3 W of the light output of the fiber laser, which is the fundamental wave, and it is difficult for it to obtain such an output of the fundamental wave exceeding 5 W. Thus, W-class high-output green and blue laser lights cannot be obtained. Further, even if the length of the fiber of the fiber laser is simply extended for the purpose of increasing the output of the fundamental wave by increasing the gain of the fiber laser, no large light output can be obtained since an absorbed amount of the fundamental wave by the fiber increases in proportion to the length.
On the other hand, the fiber length needed to be shortened in order to obtain a light having a wavelength equal to or shorter than 1070 nm since the light absorption of the fundamental wave by the fiber becomes significant as the oscillation wavelength of the fiber laser becomes shorter than 1070 nm. However, efficiency decreases if the fiber is shortened. For example, it becomes difficult to ensure high efficiency in the case of generating a light having a wavelength in the vicinity of 1030 nm.
Accordingly, there has been a problem of being difficult to obtain a W-class green laser output having a short wavelength, which should be obtained by shortening the wavelength of the fundamental wave of the fiber laser.
The aforementioned green laser is difficult to realize only by a semiconductor laser, and a method for obtaining a green laser by wavelength conversion using a wavelength conversion element made of, e.g. LiNbO3 or LiTaO3 is a mainstream at present. Further, it is known to use an infrared high-output from a fiber laser excited by a semiconductor laser as a fundamental wave to be incident on the wavelength conversion element.
On the other hand, a visible light source capable of providing a W-class high output having strong monochromaticity is necessary in realizing large-scale displays and high-luminance displays. Developments on high-output light sources are being advanced also for green light sources. Generally, a color display represents colors using light sources of three colors R, G, B and the range of representable colors can be determined in a chromaticity diagram based on the wavelengths of the R, G and B light sources. In a laser display using lasers as the respective R, G and B light sources, the color representation range can be further expanded by using the G light source having a shorter wavelength, wherefore color representation close to original colors is possible.
However, although the color representation close to original colors is made possible by shortening the wavelength of the G light source to expand the color reproduction range, there still remains the range of colors, which cannot be reproduced by any means, since only one wavelength of the G light source is determined.
On the other hand, in a laser display for representing an image by scanning a laser, the laser needs to be modulated in accordance with an image data, and an acoustooptical element and a light modulation element using LiNbO3, for example, as disclosed in Japanese Unexamined Patent Publication No. H09-246638 have been proposed for a laser modulation method.
The use of the acoustooptical element or the light modulation element using LiNbO3 is disadvantageous in terms of space for the laser modulation as well, and has an additional problem of becoming more expensive.